Thermal Staking, Swaging, or Inserting

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26 April 2008 -- A heat staking machine can be used for hot tool insertion as well as thermal staking or swaging. The most common inserts for hot tool insertion are brass female-threaded fasteners placed in molded holes to provide good purchase for bolts used to secure circuit boards or other components/subassemblies to a chassis or inside a case, or to secure the case halves. Brass inserts are preferred to steel for this operation because the copper content of the brass assures a relatively low thermal mass for the inserts and they will change temperature rapidly. The inserts are typically placed in the holes partially engaged. The press is actuated and the tips come into contact with the inserts. At this point, an insertion delay can be programmed to allow the inserts to warm up to a temperature that allows for melting of the thermoplastic material. After the delay, the press continues to press the inserts into the holes. Depth is usually controlled by a mechanical stop. After contacting the stop, the press can simply retract if the insertion is low precision, or the press can stop while compressed air is used to cool the inserts and tips to prevent the inserts floating back out of the hole on a cushion of expanding hot plastic. Getting the tip temperatures and delay times just right takes a bit of experimenting. Beware of the temptation to simply use press force to jam not-quite-hot-enough inserts into their holes, as holes almost always have suceptibility to cracking at the knit or weld lines when under hoop stress. The pull-out and torque strength of the set inserts can usually be improved by slowing the process down and allowing the heat to soak a little more deeply into the plastic during the process. As with thermal staking, inserts of various sizes can be set on multiple levels simultaneously.

10 April 2008 -- Should you call a thermal staking machine a heat staker? Most thermal staking presses actually have interchangeable tooling. Since they can be easily be converted from heat staking to thermal insertion, embossing, or degating, they should probably more properly be called thermal presses. It is possible to build a machine for the sole purpose of heat staking, in which case the term heat staker could be applied.

5 April 2008 -- Hot tool staking probably started with a screwdriver or some similar tool heated with a torch and then applied to a tab or post to capture another part. The process does not work much differently today, though the equipment and tooling has become much more sohpisticated. Most thermal staking machines today use electric coil heaters with embedded thermocoules and digital temperature controllers to maintain probe temperature. The tip, attached to the probe, is designed to have low thermal mass, that is to say it can change temperature easily. The tip is heated by the probe, which is mounted to a press that delivers the probe and tip assembly to the work and applies force to deflect the head(s) to be formed. When the tip contacts the work, it transfers heat to the work to melt the material and form the detail. If the press were simply retracted at this stage in the process, most materials would stick to the hot tip and either form strings or the head would actually be pulled off of the tab or post. To prevent this, pressurized (and sometimes cooled) air is delivered to the tip long enough to reduce the tip temperature enough to allow a clean release and leave a well-formed detail with sufficient strength to hold the assembly together. This action is usually called post-cooling. Following retraction, the tip is then reheated by the hot probe. If more than one probe assembly is installed on the machine, they can be individually controlled if there are a sufficient number of heater controllers on the machine, or they can be zoned together, with one thermocouple providing temperature feedback to the heater controller, and other heaters modulating in open-loop fashion. This works better then one might think when identical probe/tip assemblies doing the same work are on the same zone and heaters are closely matched. There is considerable freedom in tooling design for the heat staking process, and multiple size heads can be formed at multiple levels in the assembly. Probes cannot go into tight spaces in parts, though, as they may thermally deform nearby details. Likewise, not all plastics respond well to thermal staking, including matrials with sharply crystalline melting points such as polyamid (nylon), or narrow melt to degradation temperature ranges such as polyvinyl chloride.

Warning: Information in the blog is acted on at the sole risk of the reader. It is correct and accurate so far as we are able to determine, but is delivered without any warranty of any kind, including but not limited to direct or consequential damages, personal injury, or damage to product or property.

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Last updated 14 March 2018.